Compression methods are becoming increasingly important for graphics hardware architectures, since they may reduce the power used and increase performance. Compression of depth can be done in a variety of ways.
Depth offset (DO) compression is an extremely simple method for compressing depth. A tile, which is a rectangular region of depths, is compressed at a time. The minimum and maximum depths, denoted Zmin and Zmax, are found in the tile. For each depth value, one bit is stored which indicates whether the depth is encoded with respect to the Zmin or Zmax. For each depth, a residual against Zmin or Zmax is then stored.
If all the compressed residuals are small enough so they fit within the desired bit budget for the tile, then compression succeeds. Otherwise, the data may be stored in uncompressed form, or compressed using some other method.
Better compression is always desired because it can help to lower power consumption and/or may increase performance. Bandwidth can be significantly lowered when a large percentage of tiles can be compressed, and/or if a few tiles can be compressed with a high compression ratio.